1. Field of the Invention
The present invention relates to a process for preparing a novel casein powder from acid casein. The casein powder of this invention will dissolve in water to form a casein micelle having a turbidity and heat stability similar to those of a cow's milk casein micelle, and is excellent in solubility. More particularly, the present invention relates to a process for preparing said novel casein powder.
2. Description of the Prior Art
Various kinds of casein powders are available commercially, including sodium caseinate, calcium caseinate, acid casein, rennet casein, and coprecipitated casein. None of these powders, however, is entirely satisfactory. Sodium caseinate is poor in dispersibility, wettability and sinkability, and has a slight turbidity, although it is water soluble and has a very good final solubility. Calcium caseinate will separate and precipitate when allowed to stand, although it is water insoluble and has good dispersibility, wettability and sinkability. Coprecipitated casein, which has recently been developed, partially contains a milk serum protein. However, this casein is a mixture of calcium caseinate partially mixed with acid casein, and has substantially the same properties as calcium caseinate. Acid casein and rennet casein powders are water insoluble and so are dissolved in an alkaline agent and used as alkali caseinate.
None of these casein products, as described above, are soluble in water and none will provide a casein micelle which is similar to that of cow's milk.
In order for a casein powder to dissolve in water to form reconstituted cow's milk or skim milk, and to have a casein micelle similar to natural cow's milk, it should have the following properties:
1. It should be water soluble;
2. It should have good solubility. This means that it must have the following properties:
Good dispersibility (i.e., will not become lumpy readily);
Good wettability (i.e., will penetrate a water surface);
Good sinkability (i.e., large in particle size and will not float on the water surface);
Good final solubility (i.e., no final insolubles present).
3. The turbidity of a solution containing redissolved casein powder therein should be about the same as that of fresh skim milk after pasteurization. The turbidity of a solution containing dissolved common skim milk powder and diluted to 0.5% in protein concentration, is usually 1.58 to 1.64 in optical density (O.D.), read spectrophotometrically at 610 mu at 20.degree. in a photoelectric colorimeter. Therefore, the casein product should desirably show a turbidity of at least 1.20 in O.D. in aqueous solution.
4. The casein micelle of a liquid prepared by redissolving a casein powder and adjusting its protein content to 3 to 5% should have good heat stability and no protein aggregate sedimentation should be found. When centrifuging 50 ml of a solution of about 10% skim milk powder, having 3% protein concentration, at 1,000 rpm at 20.degree. C for 3 minutes in a graduated centrifugal tube, the amount precipitated is within the range of about 0.05 to 0.15 ml. Therefore, the casein should have a sufficient heat stability such that below 0.2 ml is precipitated out during the above test. A casein powder cannot be said to be good in heat stability, unless the amount of precipitate from a solution of casein powder having a protein content of about 3%, by the above test, after being subjected to a heat treatment at 120.degree. C for 10 minutes, by means of an autoclave, is below 0.2 ml.
A process is known for forming a casein micelle similar to cow's milk as described in Japanese Patent Publication Gazette No. 10900/71 by adding potassium carbonate, dipotassium phosphate, or sodium citrate to acid casein to solubilize the casein, mixing a cooled solution of calcium chloride thereto and heating to 80.degree. to 90.degree. C. The product obtained by this process, however, as evidenced by the Examples, is not a powder but is a liquid having a low turbidity of 0.4 O.D., wherein the protein concentration is adjusted to 0.5% so that the casein micelle formed cannot be similar to cow's milk. (The corresponding skim milk has a turbidity of 1.6 O.D.) Also, the powder obtained by drying the product liquid has a very low water solubility.
A need, therefore, continues to exist for a casein powder which is water soluble, has good dispersibility, good wettability and good sinkability, has a turbidity about the same as fresh skim milk after pasteurization, has good heat stability and does not exhibit protein aggregate sedimentation.